Date   : Mon, 29 Mar 1999 10:46:02 +0100
From   : "Fraser, Colin J" <Colin.Fraser@...>
Subject: Re: Running U.K. beeb on U.S. power

> -----Original Message-----
> From: Angus Duggan [mailto:angus@...]
> Sent: 29 March 1999 08:16
> To: rick_galbraith@...
> Cc: bbc-micro@...
> Subject: Re: [BBC-Micro] Running U.K. beeb on U.S. power
> 
> I'll check it out. What about the flyback sync? There is an interrupt
> generated by the 6845 (?) that indicates the start of the video frame.
> Elite and various other mode-shifting games use it to 
> determine when to alter
> the 6845 registers. The flyback signal was generated at 50Hz, I always
> assumed due to some coupling between the mains frequency and 
> video scan
> frequency (though thinking about it now, there doesn't seem 
> to be any reason
> that there should be).

Here's a bit of trivia for you...

There isn't any reason why the TV field frequency matches mains frequency,
and in fact the engineers who developed the US colour TV system had to
change the frequency chosen for American TV slightly to avoid interference
between the mains frequency and the TV signal that caused colour errors.
American TV actually runs at a frame rate of 29.97 fps.
The PAL system uses a phase change on each line in the chroma signal to
cancel out this problem (hence Phase Alternating Line). 
If you ever use video or audio equipment that syncs to a video signal, one
of the frame rates will be 30 fps 'drop frame' - a time code frame is
dropped every minute (but not every tenth minute) so that the h:m:s part of
the time code matches clock time, but there can still be an integer number
of frames per second.

Strange but true...

Colin f